There is a need for all power supplies connected to the mains to meet the harmonic limits of the European standard EN-61000-3-2 or similar in other countries. There are further needs to meet efficiency standards, e.g. 80 PLUS and Energy Star, in future. Prior arts which can meet the EN-61000-3-2 requirements can be divided into two categories.
The first category (U.S. Pat. No. 4,437,146, U.S. Pat. No. 5,134,355, U.S. Pat. No. 5,654,880, U.S. Pat. No. 6,900,623, US2006/0158912) senses the rectified AC voltage and controls the operation of the converter using a feedback loop such that the current drawn by the converter follow the rectified AC voltage. This category suffers from the problem of feedback loop stability when the AC line voltage varies over a wide range, e.g. from 115AV to 240VAC. They use complicated compensation network to ensure stability and is susceptible to noise and distortion in the rectified AC voltage. For example, the prior art U.S. Pat. No. 6,900,623 senses the RMS value of the AC line voltage and scales the loop gain accordingly.
The second category (U.S. Pat. No. 5,867,379, U.S. Pat. No. 5,742,151) uses a nonlinear carrier signal without sensing the rectified AC voltage to generate the same control signal as in the first category. This category suffers from the problem that it works best when the switching mode converter operates in the continuous mode. When the loading of the converter is only a fraction of the full load, the converter may go into discontinuous mode and the power factor of the converter can no longer be maintained at or near unity. On the other hand, the first category is free from this problem.
Both categories use Pulse Width Modulation (PWM) to control the switch inside the converter and suffer from the problem of concentrated EMI. There are studies in prior art on the merit of using Pulse Frequency Modulation (PFM) or Frequency Modulation (FM) instead of PWM control to alleviate the EMI problem such that a smaller and cheaper EMI filter can be used instead. However, these prior arts use a dedicated unit to adjust the switching frequency while the pulse width is controlled by another unit and the complexity of the design becomes double.
Lastly, in order to meet efficiency standards, e.g. the 80 PLUS and Energy Star, extra circuitry is needed to override the normal control of these power factor controllers and reduce the power consumption at light loading. This implies extra circuitry and more complex control to ensure the controllers transit smoothly between the light loading mode and normal loading mode.
The PFM approach is a well known solution to provide smooth transition between the light loading mode and normal loading mode. However, conventional single-stage AC/DC converters which do not has a Power Factor Correction front stage will suffer from the problem of larger output ripple at medium load. However, this is not a concern for two-stage AC/DC converter which has a Power Factor Corrector as the front stage and a DC/DC converter as the second stage.
Thus, there is a need to combine the functions of power factor correction and PFM into a single controller to overcome all problems in the prior arts.